Lithographic printing is based upon the immiscibility of oil and water, in which the oily material or ink is preferentially retained by the image area and the water or fountain solution is preferentially retained by the non-image area. When a suitably prepared surface is moistened with water and an ink is then applied, the background or non-image areas retain the water and repel the ink while the image areas accept the ink and repel the water. The ink on the image areas is then transferred to the surface of a material upon which the image is to be reproduced, such as paper, cloth and other materials. Commonly, the ink is transferred to an intermediate material called the blanket which in turn transfers the ink to the surface of the material upon which the image is to be reproduced.
A widely used type of lithographic printing plate has a light-sensitive coating applied to an aluminum support. The coating may respond to light by having the portion that is exposed become soluble so that it is removed in the developing process. Such a plate is referred to as a positive-working printing plate. When the exposed portion of the printing plate becomes insoluble, the plate is referred to as a negative-working plate. In each instance, the image areas remaining are ink-receptive or oleophilic and the non-image areas or background are water-receptive or hydrophilic. The differentiation between image and non-image areas is made in the exposure process where a film is applied to the plate under a vacuum to insure good contact. The plate is then exposed to a light source, a portion of which is composed of ultraviolet radiation. In a negative-working plate, the areas on the film corresponding to the image areas are clear, allowing light to harden the image area coating, while the areas on the film corresponding to non-image areas are opaque, so the areas not struck by light can be removed during development. The light-hardened surface of a negative-working plate is oleophilic and will accept ink. The non-image areas, which have had the coating removed through the action of a developer, are hydrophilic.
Negative-working lithographic printing plates comprising a radiation-sensitive composition that includes a photocrosslinkable polymer containing the photosensitive group --CH.dbd.CH--CO-- as an integral part of the polymer backbone are well known. See, for example, U.S. Pat. No. 3,030,208 (Schellenberg et al.), U.S. Pat. No. 3,622,320 (Allen), U.S. Pat. No. 3,702,765 (Laakso) and U.S. Pat. No. 3,929,489 (Arcesi et al.).
Colorants have been used in negative-working photosensitive compositions to provide a visible image for effective inspection of processed plates. Typical colorants are water-insoluble pigments having a broad spectrum of high-density colors. Use of such pigments may present problems because the pigment dispersions are difficult to make and are not always stable in photosensitive compositions. In severe instances, the pigments aggregate and come out of solution, resulting in a phenomenon known in the art as "shocking". This destroys the entire photosensitive coating.
In contrast, some dyes can be used that are soluble in the photosensitive coating and are thus incapable of "shocking", but the dyes must be carefully selected for compatibility with the polymers that are used. However, because the dyes are soluble in the coating solvents, they cannot be suitably filtered to remove impurities, and when the plates are developed, the developer solution becomes dirty along with the processor itself.
Whether a dye or pigment is used in the photosensitive composition, a problem remains. The colorant tends to leach from the printing plate image because the colorant is a separate component within the composition. This problem arises because of the routine changing of developer solutions and the change in developer solution strength with use and time. In the printing industry, it is common practice to interchange products so that one manufacturer's plate can be processed in another manufacturer's developer solution. In some cases, the developer solution is too inactive for developing the plate for another manufacturer. In other cases, the developer solution may be too active for the specific printing plate, attacking the imaged layer and causing leaching of colorant into the developer solution. In some instances, partial removal of the colorant may lead to the incorrect assumption that the developer solution is attacking the imaged layer when it is not doing so.
Background stain is another common problem attributable to the use of a colorant component in the photosensitive. After the printing plate has been processed, it is not uncommon to see discoloration or tinting of the background of non-image areas. This may be caused by an insufficiently active developer solution, aging of the coating, or simply the staining of the substrate by the colorant. In any case, it is undesirable because it suggests that the printing plate may have or exhibit performance problems.
Thus, a need exists for a negative-working photo-sensitive composition and printing plate that has an appropriate color but which does not exhibit the problems noted above.